Year	Years
1970	3.2
1971	3.4
1972	3.5
1973	3.6
1974	3.7
1975	3.8
1976	4
1977	4.1
1978	4.2
1979	4.3
1980	4.5
1981	4.6
1982	4.7
1983	4.9
1984	5
1985	5.1
1986	5.3
1987	5.4
1988	5.5
1989	5.7
1990	5.8
1991	5.9
1992	6.1
1993	6.2
1994	6.3
1995	6.5
1996	6.6
1997	6.7
1998	6.9
1999	7
2000	7.1
2001	7.3
2002	7.4
2003	7.5
2004	7.6
2005	7.8
2006	7.9
2007	8
2008	8.2
2009	8.3